1. Field of the Invention
This invention relates to a method for the production of a sialon material and a silicon carbide material with improved properties obtained by forming a phase of closely packed crystals in the surface region. More particularly, this invention relates to a method for the production of a sialon material and a silicon carbide material exhibiting a notable improvement in such qualities as oxidation-resistance, resistance to heat, resistance to thermal shock, corrosion-resistance, and mechanical strength by giving a specific surface treatment to respective raw materials thereby giving rise to a phase of closely packed crystals in the surface region of raw material.
2. Prior Art Statement
Sialon is a compound consisting of silicon, aluminum, oxygen, and nitrogen atoms as represented by the following formula: EQU Si.sub.6-z Al.sub.z O.sub.z N.sub.8-z ( 1)
(wherein z stands for 0-4).
Sintered articles of this compound excel in resistance to heat, resistance to thermal shock, and mechanical strength and, what is more, exhibit highly desirable corrosion-resistance to molten aluminum, copper, and iron, for example, and therefore are expected to find growing utility as structural materials.
These sintered article of sialon can be manufactured relatively easily, for example, by the hot press method which comprises mixing silicon nitride powder with aluminum nitride powder and aluminum oxide powder and heating the resultant mixed powder at elevated temperatures under high pressure. Recently, sintered articles of sialon possessed of highly desirable properties are produced by the normal-pressure sintering method.
Sintered articles of silicon carbide excel in heat-resistance, corrosion-resistance, and mechanical properties such as rigidity, bending strength, and modulus of elasticity. Efforts are being made to promote utility of the sintered articles of silicon carbide as heat-resistant structural materials for use in gas turbines and automobile engines, for example. They are expected to enjoy expanding utilization in the future.
The sintered articles of silicon carbide, however, entail a weak point common to all non-oxides, i.e. a serious drawback that they are liable to be oxidized in the air at elevated temperatures and that, particularly in the presence of steam, they are corroded to form a vitreous texture containing pores in the surface region and suffer from degradation of physical properties. For the elimination of this drawback, there have been proposed various methods such as, for example, the normal-pressure sintering method which comprises mixing silicon carbide powder with a specific metal and a carbon and heating the resultant mixture at an elevated temperature to effect reaction, the reaction sintering method which comprises mixing silicon carbide powder with carbon and feeding silicon to the reaction system from outside, and the hot press method which comprises mixing silicon carbide powder with a catalyst and heating the resultant mixture at an elevated temperature under high pressure.
The sintered articles of sialon and silicon carbide which are produced by the methods mentioned above are such that their sections are found to contain numerous pores measuring approximately in the range of 1 to several microns in diameter.
When a sintered article contains such pores, it is liable to undergo oxidation in the air at an elevated temperature and incur various undesirable phenomena. For example, the oxygen in the air diffuses through these pores in the sintered article to form a thin amorphous film of silicon dioxide first at a temperature of about 700.degree. C. and, then at a still higher temperature of 1,200.degree. C., induce formation of cristobalite crystals. In the sintered article of sialon, low melting oxide products comprising a mullite phase, a vitreous phase and a phase of grain boundary impurity aggregate occur and, in the sintered article of silicon carbide, silicates and glass containing pores and cracks occur respectively in the surface region. As a result, the sintered articles suffer from degradation of physical properties and incur occurrence of cracks.
When the sialon material and the silicon carbide material do not possess a compact texture but contain pores as described above, they are deficient in resistance to oxidation at elevated temperatures and have only limited utility as heat-resistant structural materials intended for exposure to elevated temperatures. It is, therefore, considered necessary to impart high-temperature oxidation-resistance to the sialon material and the silicon carbide material by allowing the materials to form a compact protective layer in the surface region thereof.
No method has yet been developed which enables an oxidation-resistant protective layer exhibiting highly desirable adhesiveness to a substrate and possessing a thermal expansion coefficient approximating that of the substrate to be formed uniformly and inexpensively on the surface of a substrate, particularly on a substrate with a large surface area and a complicated shape.